MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 453 



in abumlauce around the retiuular elements. This is without doubt ectodermic iu origin. Some 

 yellow pigment also occurs below the basement membrane. This may be ectodennic or meso- 

 dernaic, but the great bulk, if not all the accessory pigment cells, which in the adult eye extend 

 their processes through the basement membrane, are ectodennic in their origin. 



At the edge of the retinal plate the cells become very much elongated (Pigs. 146, 1S8), and 

 finally can no longer be distinguished from the superficial ectoderm. The thickening of the retinal 

 plate is due solely to emigration. This might be inferred by the interwedgiug of the nuclei (Figs. 

 188, 189), and it is proved in cases of cell division by the position of the equatorial plate, which is 

 always perpendicular to the surface.* 



A later stage in the development of the eye is illustrated by Figs. 190 and 191, which are 

 anterior (superficial) and posterior (deep) transverse sections. In Fig. 190 we see the retina differ- 

 entiated into cell clusters. Each cluster represents part of an otumatidiuin the corneal hypo- 

 dermis, the cone cells, and possibly the distal retinular cells. The lower stratum of small nuclei 

 belongs to the proximal retinulre and to the accessory pigment cells. In deeper section, Fig. 191, 

 we distinguish the proximal retiuulse black, rod-shaped bodies piercing the basal membrane. 

 The nuclei lie at the distal extremity of the pigment. Between the pigmented parts of the retiuular 

 cells other nuclei occur, which possibly represent an eighth rudimentary retiuular cell (rtl 1 ). At the 

 surface of the eye there is a stratum of cells with elongated nuclei. These are the nuclei of the cor- 

 neal cells and probably also of the distal retinulte. Between this and the inner stratum of retinular 

 nuclei, the nuclei of the cone cells are seen, and a granular substance which is probably the first 

 trace of the peculiar secretion of these cells. In an older stage represented by the drawing (Fig. 

 192) we see all these parts iu a higher degree of development, and the eye has grown out into a 

 very prominent lobe. Fig. 167 is intermediate between this and Fig. 191. The central parts of 

 this eye are the most highly developed, and as we pass to the periphery especially away from the 

 middle line, the ommatidia are less and less developed, until they reach the condition of a single 

 layer of uudifferentiated ectoderm. Thus, in a single section (like Fig. 192), we have a sort of com- 

 posite picture of the various stages through which the developing retina has passed. 



A considerably older stage is reached in Fig. 194. The cells of the corneal hypodermis are 

 quite large and have already secreted a cuticular lens. The nuclei of the cone-mother cells are 

 also conspicuous. 



Later still, when the embryo is nearly ready to hatch, the eye has undergone very slight 

 change. Fig. 187 represents an oblique longitudinal section through the eye stalk of Alpheus 

 heterochelis. The black pigment of the retinular cells has been removed by the action of the nitric 

 acid. Irregular sheets and masses of yellow pigment occur both above and below the basement 

 membrane. The conspicuous stratum of nuclei, in which the bases of the cone cells aie embedded, 

 belong to the proximal retinular cells. The nuclei (unrepresented) lie close to the surface between 

 the corueal cells. The cone cells end distally in a conical cap of protoplasm, the apex of which 

 touches, in some cases at least, the corneal facet. The proximal ends of the coute cells taper gradu- 

 ally and can not be traced below the pigment zone. 



In the first larva of Alpheus saulcyi the structure of the eye is similar. This is illustrated in 

 Figs. 201-204 and Fig. 209. In transverse section (Fig. 201) the corneal cells are crescent-shaped. 

 The distal retiiiuhc lie in the same plane with the latter and have the peculiar arrangement shown 

 in the drawing. They are grouped in pairs, so that each set of corneal cells is surrounded by 

 six nuclei two pairs of nuclei and two single nuclei. The two odd nuclei pertain to the omuia- 

 tidiuin in question. The delicate membranes which appear to surround the corneal cells belong, 

 in all probability, to the distal retiuular cells. 



The structure of the compound eye has become a favorite subject of research during the past 

 five years, and the important study of the development of the faceted eye, about which very little 

 was known when Balfour's "Comparative Embryology" appeared, has not been neglected; still 

 much work needs yet to be done iu this direction. The literature of this subject has been recently 

 examined by Parker (48), and I will therefore add to ihis account only a few comparative notes. 



"Parker states that th conical hypoderniis arises in the lobster by simple dehunination. (Op. tit.) I have 

 never seen delamiuating cells iu any part of the retina of Alpheus or Paheniouetes. 



